/Engine/Source/Runtime/Engine/Public/FieldNotification/FieldNotificationLibrary.h
The Field Notification system allows a user to know when a property value is modified at runtime (note that it can be a function return value). The class needs to implement theINotifyFieldValueChangedinterface. The property' setter usually follows this pattern "if (new value != old value) assign the new value; broadcast that the value changed;". The Blueprint implementation handles that setter logic automatically with SetPropertyValueAndBroadcast. When a property value is modified by replication, the RepNotify will call BroadcastFieldValueChanged. A function can also be a Field Notify. The function needs to be const and return a single value.
class UFieldNotificationLibrary : public [UBlueprintFunctionLibrary](API\Runtime\Engine\Kismet\UBlueprintFunctionLibrary)